A pressure sensor or a barometer is an instrument that is used to measure atmospheric pressure. Atmospheric pressure is the pressure above any area in the Earth's atmosphere and it is caused by the weight of air. Atmospheric pressure decreases with increasing altitude as there are fewer air molecules as elevation increases.
The term atmospheric pressure refers to the pressure generated by the weight of the air surrounding the earth. The pressure as a function of altitude is not a linear function due to the compressibility of air; the atmosphere is denser at lower altitudes. The atmosphere is also not uniform, there are mounds and valleys that create high and low pressure areas. A barometric reading used by meteorologists measures the atmospheric pressure and is typically normalised to sea level. An absolute pressure transducer converts the normalised sea level pressure to an atmospheric pressure at a current altitude. The conversion can be done if the normalised sea level pressure and current altitude are known.
Pressure altitude is commonly used expression when referring to the vertical heights of objects in the atmosphere above the Earth's surface. Pressure altitude, which is actually an indication of the ambient pressure, is expressed in terms of the altitude at which that pressure would exist on a standard day defined by International Standard Atmosphere (ISA).
Furthermore, atmospheric pressure conditions vary widely on Earth, and these variations are important in studying weather and climate. Atmospheric pressure also shows a daily rhythm. This effect is very strong in tropical zones, whereas it is almost zero in polar zones. Barometers can be divided into two different groups: liquid and aneroid barometers. Liquid barometers can further be divided into water-based and mercury barometers.
Global navigation satellite systems (GNSS) can be used to determine location of the receiver. Examples of such systems are American GPS, Russian GLONASS and European Galileo of which GPS and GLONASS are currently operating and Galileo is planned to be in full operation in 2008. One drawback with the global navigation satellite systems is that altitude measurements are inherently less accurate than the horizontal measurements. The reason for this is geometry, namely in order to obtain accurate altitude measurements, satellites should be nearly overhead. Unfortunately this is not often the case.
European patent application EP1154231 by LUCENT TECHNOLOGIES INC., filed 20 Nov. 2000 discloses a communication device and method for estimating a more accurate vertical position or altitude of a communication device using atmospheric pressure measurements. A communication device measures local atmospheric pressure using a pressure sensor at a communication device and estimates an altitude of the communication device using the local measured barometric pressure and atmospheric pressure model information sent, for instance, by a base station. Atmospheric pressure model information corresponds to another atmospheric pressure measurement at a known elevation near the communication device.
A problem arises, if a user of the communication device moves frequently or the weather changes rapidly. According to prior art solutions, the user of the communication device should frequently download the latest atmospheric pressure model onto their phones so that the calibration information is up-to-date in changed weather conditions.